Catecholestrogens were found to have a high affinity for estrogen receptors of brain, uterus, mammary tissues and liver. Preliminary results using a mammary carcinoma cell line in vitro indicate that catecholestrogens have an inhibiting effect on cell growth. Catecholestrogens can compete with estrogen receptors in these tumors. Both the catechol-forming enzyme and its inactivating enzyme, catechol-O-methyltransferase, are present in human breast tissues. A highly significant increase in catechol-O-methyltransferase activity was found in malignant breast tissue. Estrogen has been found to release endogenous noradrenaline and dopamine from certain areas of the rat hypothalmus.